On motor vehicles with pneumatic springs, a load-controlled level control os generally used. The quantity of air in the spring bellows of the pneumatic spring is thereby regulated as a function of the vehicle load, so that a specified distance is always present between the vehicle body and the vehicle axle.
An arrangement of the type described above is disclosed in the published German patent application No. DE-OS 30 31 453, which corresponds to U.S. Pat. No. 4,354,693. This apparatus of the prior art consists essentially of the following parts.
In practice, a distance measurement apparatus measures the distance of the vehicle body from the vehicle axle, which changes as the vehicle is loaded or unloaded. The measurement data are compared with a specified value, namely, the standard level by means of an electronic evaluation circuit. When a deviation occurs, the standard level is re-established by charging or discharging of the pneumatic springs. During pressurization, the electronic system switches a pneumatic spring valve between the pressure medium source and the pneumatic springs to the open position, and causes the pressure medium source to pump air from the pressure medium source into the pneumatic springs, until the electronic system recognizes the standard level indicated by the distance measurement apparatus. The flow of compressed air is then shut off, and the pneumatic spring valve is closed. The discharging occurs when the electronic system opens the pneumatic spring valve, and a second multi-way valve is open to the atmosphere. The pneumatic springs are then discharged into the atmosphere until the electronic system recognizes the standard level and closes both valves.
An air dryer integrated into the apparatus of the prior art is located so that, during pressurization, it carries the flow in one direction, and during discharge, it carries the flow in the opposite direction. During the discharge period, the moisture captured and collected in the air dryer is discharged into the atmosphere via the discharge valve located behind the outlet of the air dryer. The discharge valve is electromagnetically switched by the electronic system. It has been found that valves like those in question, which are designed to remove moisture, have a tendency to ice up and consequently become inoperable at low temperatures. This causes a particular disadvantage for magnetically-controlled valves, since there is only limited space available for installation and the limited power of the magnets, which results from even minor icing the force of the magnets to be restricted. Thus, the cross-sectional opening for the flowing medium is limited, so that proper operation can be a problem.
Another disadvantage of the previous apparatus is that, when the vehicle is jacked up and the pneumatic spring system is operational, the distance measurement apparatus can simulate a non-existent unloading to the electronic system. Under this condition, the electronic system maintains the connection between the pneumatic springs and the atmosphere, which can result in a complete discharging of the pneumatic springs. The resumption of vehicle operation with such completely discharged pneumatic springs can lead to damage to the bellows of the pneumatic springs, until the level control system resumes functioning.